Chaff, originally called Window by the British, and Düppel by the Second World War era German
Luftwaffe, is a radar countermeasure in which aircraft or other targets spread a cloud of
small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears
as a cluster of primary targets on radar screens or swamps the screen with multiple returns.
Modern armed forces use chaff to distract radar-guided missiles from their targets.
Most military aircraft and warships have chaff dispensing systems for self-defence. An intercontinental
ballistic missile may release in its midcourse phase several independent warheads as well
as penetration aids such as decoy balloons and chaff.
Chaff can also be used to signal distress by an aircraft when communications are not
functional. This has the same effect as an SOS, and can be picked up on radar. It is
done by dropping chaff every 2 minutes.
Second World War The idea of using chaff developed independently
in the United Kingdom, Germany and the United States.
In 1937, British researcher Gerald Touch suggested that lengths of wire suspended from balloons
or parachutes might overwhelm a radar system with false echoes and R. V. Jones had suggested
that pieces of metal foil falling through the air might do the same.
In early 1942, a Telecommunications Research Establishment researcher named Joan Curran
investigated the idea and came up with a scheme for dumping packets of aluminium strips from
aircraft to generate a cloud of false echoes. An early idea was to use sheets the size of
a notebook page; these would be printed so they would also serve as propaganda leaflets.
However, it was found the most effective version used strips of black paper backed with aluminium
foil, exactly 27 by 2 centimetres and packed into bundles each weighing 1 pound. The Head
of the TRE, A. P. Rowe, code-named the device "Window".
Meanwhile in Germany, similar research had led to the development of Düppel. The German
code name was that of the estate on which the first German tests with chaff had been
made, circa 1942. Once the idea had been passed to the US, Fred Whipple developed a system
for dispensing strips for the USAAF, but it is not known if this was ever used.
The systems were all essentially identical in concept: small aluminium strips cut to
one-half of the target radar's wavelength. When hit by the radar, such lengths of metal
resonate and re-radiate the signal. Opposing defences would find it almost impossible to
distinguish the aircraft from the echoes caused by the chaff. Other radar-confusing techniques
included Mandrel, Piperack, and Jostle. However, unaware of the opposing air force's
knowledge of the chaff concept, planners felt that using it was even more dangerous than
not, since, as soon as it was used, the enemy could easily duplicate it and use it against
them. In particular, the British government's leading scientific adviser, Professor Lindemann,
balefully pointed out that if the RAF used it against the Germans, the Luftwaffe would
quickly copy it and could launch a new Blitz. This caused concern in RAF Fighter Command
and Anti-Aircraft Command, who managed to suppress the use of Window until July 1943.
At this time, it was felt the new generation of centimetric radars available to Fighter
Command would deal with any Luftwaffe response to RAF Bomber Command use.
Examination of the Würzburg radar equipment brought back to the UK during Operation Biting
and subsequent reconnaissance revealed to the British that all German radars were operating
in no more than three major frequency ranges, and thus were prone to jamming. "Bomber" Harris,
Commander-in-Chief of RAF Bomber Command, finally got approval to use Window as part
of Operation Gomorrah, the fire raids against Hamburg.
The first aircrew trained to use Window were 76 squadron. Twenty-four crews were briefed
on how to drop the bundles of aluminised-paper strips, one every minute through the flare
chute, using a stopwatch to time them. The results were spectacular. The radar guided
master searchlights wandered aimlessly across the sky. The anti-aircraft guns fired randomly
or not at all and the night fighters, their radar displays swamped with false echoes,
utterly failed to find the bomber stream. A vast area of Hamburg was devastated, resulting
in more than 40,000 civilian casualties, with the loss of only 12 bombers. Squadrons quickly
had special chutes fitted to their bombers to make the deployment even easier. Seeing
this as a development that made it safer to go on operations, many crews got in as many
trips as they could before the Germans found a countermeasure.
Although the metal strips puzzled the German civilians at first, German scientists knew
exactly what they were because they had developed Düppel themselves but had refrained from
using it for exactly the same reasons as Lindemann had pointed out to the British. Thus for over
a year the curious situation arose where both sides of the conflict knew how to use chaff
to jam the other side's radar, but refrained from doing so fearing that if they did so
the other side would 'learn the trick' and use it against themselves.
The use of Window rendered the ground-controlled 'Himmelbett' fighters of the Kammhuber Line
unable to track their targets in the night sky and left radar-guided guns and spotlights
useless. In response to this, a new tactic, called Wilde Sau, or 'Wild Sow', was developed
by Oberst Hajo Herrmann to cope with the lack of accurate ground guidance, and led to the
creation of three new fighter wings dedicated to these tactics, numbered JG 300, JG 301
and JG 302. Ground operators would radio-direct single seat fighters and night fighters to
areas where the concentrations of chaff were greatest, and allow the fighters to visually
acquire their targets, often against the fires and searchlights below. A few of the single
seat fighters used by these new wings had special installations of the FuG 350 Naxos
radar detection gear to spot British bombers at night, homing in on the British bombers'
H2S bomb-aiming radar emissions. A lesser known fact is that the Luftwaffe
used this technology just six weeks after the above-mentioned Hamburg raid. The German
strips were cut into 80 centimetre by 1.9 centimetre lengths and first dropped during
a raid on 7–8 October 1943. In a series of raids in 1943, and the 'mini-blitz' of
Operation Steinbock between February and May 1944, Düppel allowed German bombers to once
again attempt to operate over London. Although theoretically effective, the small number
of bombers, notably in relation to the RAF's now-large night fighter force, doomed the
effort from the start. The British fighters were able to go aloft in large numbers and
often found the German bombers in spite of their Düppel.
Chaff in the United States was co-invented by astronomer Fred Whipple and Navy engineer
Merwyn Bly. Whipple proposed the idea to the Air Force who he was working with at the time.
However, initial tests were unsuccessful as the foil strips stuck together and fell as
clumps for little or no effect. Bly solved this by designing a cassette such that the
strips rubbed against it as they were expelled, gaining an electrostatic charge. Since the
strips all had a similar charge they repelled each other, enabling the full countermeasure
effect. After the war, Bly received the Navy Distinguished Civilian Service Award for his
work. Falklands War
British warships in the Falklands War made heavy use of chaff.
During the war, British Sea Harrier aircraft lacked their conventional chaff-dispensing
mechanism. Therefore Royal Navy engineers designed an impromptu delivery system of welding
rods, split pins and string, which allowed six packets of chaff to be stored in the airbrake
well and be deployed in flight. It was often referred to as the "Heath Robinson chaff modification",
due to its complexity. See also
Anti-aircraft Anti-ballistic missile
Countermeasure Infrared countermeasures
Electronic countermeasures Flare
Notes
References Goebel, Greg. The Wizard War: WW2 & The Origins
Of Radar v.2.0.2, retrieved 2008-03-18 Jones, R. V.. Most Secret War: British Scientific
Intelligence 1939–1945. Hamish Hamilton, London. ISBN 0-241-89746-7
External links BBC: The History of Radar
Obituary of Joan Curran in The Independent, Feb 19, 1999 by Tam Dalyell
Window, The History of Sun Engraving and Sun Printers